This invention is directed generally to the fastener arts, but specifically to sealing fasteners having an undercut groove or channel in the underside of a fastener head for accommodating a sealing element, (specifically an o'ring type elastomer) to accomplish sealing engagement with a workpiece having a threaded or unthreaded aperture.
In general sealing fasteners are well known in the art, spurred on by the space age when finding new ways to seal fasteners became a primary focus. Outdated methods such as copper washers, rtv sealant, etc. are still used to seal fasteners in some applications; however, as the sophistication our world increases, the need for reliable methods of sealing fasteners also becomes increasingly more crucial. That is why many of these inferior methods of sealing are gradually being phased out and replaced with more reliable sealing methods. One of the best ways to accomplish this task is to provide a formed groove or channel in a normally flat undersurface of the fastener head to accommodate a sealing element that is held captive in the fastener head, also achieving metal to metal contact with the workpiece and the outer rim of the fastener head. However, all previous designs have not properly calculated the groove in the fastener head. This causes sealing element failure. In static sealing threaded fastener designs, it is crucial that the groove be precisely calculated in depth, volume, angle, and configuration if one hopes to maintain a positive "seal line" between the sealed surfaces. Without a precisely calculated groove design, the sealing element will either compress too much or not compress enough. For example, by using too large a sealing element it will not have enough volumetric space to accommodate it and will, therefore, force the excess volume of the sealing element beyond the groove area, causing the sealing element to extrude and pinch between the screw and the workpiece in a process known as extruding "on the take down face". Another problem associated with previous designs is a process known as compression set. A sealing element must maintain a continuous "seal line" between the sealed surfaces. The establishment of this "seal line" is a function of groove design and sealing element cross section which determines the proper amount of squeeze (compression) on the sealing element. When a sealing element volume is larger than the area sealed, it causes excessive squeeze on the sealing element. This excessive squeeze causes sealing element deformation and loss of seal integrity, therefore rendering the sealing element ineffective. A third problem with previous designs is a process known as installation damage. As the fastener is being assembled to the workpiece, the excessive compression of the sealing element causes it to stick between the end wall surface of the groove in the fastener head and the workpiece, thereby twisting and deforming the sealing element and/or causing sealing element extrusion as previously mentioned. When too small a sealing element is used, there is not enough compression on the sealing element to maintain a continuous "seal line" between the sealed surfaces rendering its sealing capabilities useless. As an additional matter, it is vital that fasteners of this type be cold formed without removal of material from the shank or head portion of the fastener since an alteration of this type weakens the grain flow structure of the fastener in a high stress area and greatly increases the chances of head separation either before or after the fastener is tightened to its proper torque specification. It is extremely important that these fasteners maintain the ability to withstand the stress involved when tightened to normal torque values. The main reason for a modification of this type is that during cold forming or roll forming threading operations there is generally an external screw thread of up to one and one half thread pitches of incomplete thread between the undersurface of the fastener head to where the thread begins on the fastener shank. This unthreaded portion would normally keep the mating surfaces from achieving adequate metal to metal contact thus preventing a positive seal. However, using a smaller diameter cold forming wire than is normally used when manufacturing similar products of the same diameter affords the flexibility necessary to maintain high quality while forming the fastener to the minimum pitch diameter. This in conjunction with limiting the unthreaded length from the head to a maximum of 1 incomplete thread assures a complete metal to metal engagement with a workpiece having a standard size threaded or unthreaded aperture. This eliminates the need for any alterations to the fastener as mentioned above and thereby maintains fastener integrity.